1. Field of the Invention
The invention relates to an implantable ventricular heart stimulator for stimulating the right or left or both ventricles of the heart. Such a heart stimulator is known as a single-chamber heart stimulator or as a biventricular heart stimulator and may also include the function of an implantable cardioverter/defibrillator (ICD).
2. Description of the Related Art
Implantable heart stimulators in the form of heart pacemakers or cardioverter/defibrillators are fundamentally known. Such heart stimulators are usually connected to electrode lines which have stimulation electrodes or defibrillation electrodes in one ventricle of the heart or in the immediate proximity thereof. A heart pacemaker may deliver an electric stimulation pulse to the myocardium of a heart chamber via a stimulation electrode to thereby induce a stimulated ventricular contraction if the stimulation pulse is of a sufficient intensity and if the myocardial tissue is not just then in a refractory phase. To trigger a stimulated ventricular contraction in this way, electrode lines having relatively small-area stimulation electrodes are usually used because for triggering a stimulated ventricular contraction, it is sufficient if only a small portion of the myocardium of this ventricle is stimulated initially. Within the context of this description, such a stimulated ventricular contraction is referred to as a stimulated event. If a natural ventricular contraction occurs, this is referred to as intrinsic action or an intrinsic event within the context of this description. A contraction of the right atrium of the heart, for example, is known as an atrial event, which may be a natural atrial event or, in the case of an atrial pacemaker, a stimulated atrial event. In the same sense, natural (intrinsic) and stimulated left ventricular and right ventricular events may also be differentiated.
With regard to the stimulation of a heart chamber, in particular the right or left ventricle of the heart, it should also be pointed out that the stimulation is performed in synchronization with the atrium in order to reproduce as well as possible the natural contractile frequency of the heart in which the right atrium contracts first and then, after an atrioventricular conduction time, the right ventricle contracts, while at the same time or slightly later the left ventricle contracts. Under certain circumstances, the natural ventricular contraction subsequent to a natural contraction of the atrium fails to occur. In typical biventricular pacemakers, the natural contraction of the atrium is therefore detected as an intrinsic atrial event and the right and/or left ventricle is/are stimulated after a predefined conduction time.
Such natural (intrinsic) events are detected by deriving electric potentials of the myocardial of the respective ventricle with the help of sensing electrodes, which are part of a corresponding electrode line. The sensing electrodes may at the same time be stimulation electrodes and may be used alternately as stimulation electrodes and as sensing electrodes. Typically an electrode pair consisting of a tip electrode and a ring electrode is provided for sensing, the tip electrode also serving as a stimulation electrode. The sensing and stimulation in the ventricle are performed with the help of a ventricular electrode line and the stimulation and sensing in the atrium (in the right atrium) are performed using an atrial electrode line which is separately connected in the respective heart stimulator. In addition, a left ventricular electrode line may also be provided, typically passing through the coronary sinus and a lateral vein branching off therefrom into proximity to the left ventricle and may have a small-area stimulation electrode and/or sensing electrode there.
The typical stimulation modes which are implementable with a heart stimulator may be assumed to be known (VVD, DDD, etc.) so they need not be explained further here.
Beyond the properties of a heart pacemaker already described here, of delivering to the heart a stronger current pulse, which should not only stimulate (depolarize) a small portion of the myocardium but should depolarize the largest possible amount of myocardium and thus make it refractory to thereby interrupt the typical cycling stimulation of the myocardium that is typical of fibrillation. Such a pulse is known as a defibrillation shock. It is typically delivered via a large-area defibrillation electrode in comparison with the stimulation electrode or sensing electrode.
This is often implemented in the form of a shock coil on the outer surface of the electrode line in the respective chamber of the heart. For example, a ventricular electrode line in addition to a tip electrode or a ring electrode for stimulation and sensing may also have a ventricular shock coil as well as a proximal shock coil situated in the superior vena cava after implantation.
A defibrillation shock is usually delivered when the heart stimulator detects a fibrillation, i.e., an irregular high-frequency intrinsic activity of the heart which does not lead to complete contraction of the respective chamber of the heart. Such a fibrillation is classified as a tachycardiac arrhythmia, which includes tachycardias in addition to fibrillations. In contrast with fibrillation, complete contraction of the respective chamber of the heart occurs regularly in tachycardia but at a higher rate than would be physiologically appropriate. Such tachycardias can often be treated by antitachycardiac stimulation and do not require a defibrillation shock. Fibrillations are usually treated with a defibrillation shock.
In ventricular tachycardias, a distinction is made between supraventricular tachycardias (SVT) and ventricular tachycardias (VT) in the narrower sense. The latter originate in the ventricle itself, whereas supraventricular tachycardias originate in the atrium. For the treatment initiated after detection of tachycardia, the type of ventricular tachycardia (ventricular tachycardia in the narrower sense (VT) or supraventricular tachycardia (SVT)) is important.